1. Field of the Invention
This invention relates to shaking devices for flasks and other containers holding liquid mixtures which are used in biological and other applications, and more particularly to reversing rotatory shakers.
2. Description of the Prior Art
Shakers have been employed for many years for a variety of applications in the chemical, biological, metallurgical, and other industries. Shakers impart a mixing motion to a liquid which is contained in one or more flasks. The flasks, usually made of glass or a polymer, are attached to a platform on the top of the shaker. Because the flasks are so located, they are often referred to as "shake flasks".
Shakers are sometimes enclosed in an environmental chamber held at a controlled temperature. The chamber often becomes an integral part of the shaker, but the device may also be placed in a controlled temperature room. Special lights may be incorporated in the shaker chamber when photosynthesis is desired.
One common application of shakers is to ensure that all components of the liquid are in intimate contact and are well mixed. Another common application is to effectuate mass transfer between liquid and gas by maximizing the boundary contact between these components. For this use, the shake flasks usually comprise Erlenmeyer flasks or some variant of Erlenmeyer flasks. The flask openings are covered with any of a variety of well-known closures which permit free exchange of gases between the flask and the ambient environment. For example, many biological applications require the transfer of oxygen from air to a liquid medium to encourage the growth of microorganisms. Simultaneously, carbon dioxide, a product of metabolism, must be removed from the liquid to prevent accumulation of this gas and inhibition of the growth process.
Prior art shakers accomplish these gas exchanges by imparting an external force to the flask, which in turn creates a vigorous shaking action in the liquid. Most commonly, the shaking action results from the shaker platform being moved in a circular pattern. Increasing the energy applied to the shaker platform increases the rotational speed of the platform and the vigorousness of the shaking action. Regardless, no matter how vigorous the shaking action, the entire body of fluid in the flask still rotates in a single angular direction around the center axis of the flask. Consequently, the motion of one element of liquid relative to another is poor. In fact, due to inertial and frictional considerations, the entire body of fluid tends to rotate as a unit on the walls of the flask. Thus, mixing is accomplished entirely by viscous action, and at high fluid viscosities, the quality of mixing may become quite poor. In short, large amounts of energy must be expended to realize the desired rate of gas exchange in a rotatory shaker.
Various tactics are currently employed to overcome this mixing limitation. One approach involves placing indentations on the walls of the flasks; another involves placing baffles in the flasks. These tactics are often helpful, but they do not solve the underlying problem, which is the mixing limitations inherent in the shaker's basic pattern of motion.
This mixing limitation has been addressed by utilizing a shaker platform motion that is a simple back-and-forth or reciprocating action. Reciprocating shakers can sometimes yield good mixing. However, these reciprocating designs suffer from excessive splashing even at relatively low shaker rates. This splashing is often unacceptable because it results in excessive spillage (from open containers) or contamination (from touching the stopper in closed containers).
Furthermore, there exists a fundamental limitation in these prior art shaker designs that cannot always be overcome either by more vigorous rotatory shaking or tolerating the inherent spillage/contamination in reciprocating shaking. In some instances, it is not possible to exchange gases rapidly enough, and the flask becomes "mass transfer limited". Such operations are not reproducible. Thus, they may yield unreliable results.
An improved method of shaking which would overcome this and other problems associated with present shaker designs is desirable.
In short, prior art shaker designs offer either a circular motion or a linear reciprocating motion. The motion is always in a plane, and the plane is usually, but not necessarily, horizontal. All current designs frequently run at the limits of their capabilities.